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52                                 CHEMICAL ENGINEERING
efficiency is variable. While the group drive involves losses due to friction of shaft bearings, the individual motor is apt to operate at a poor load factor and therefore at low efficiency. Where one machine is likely to be operated alone frequently, it may well have its own motor. The group drive does not permit of electrical methods of speed variation. Individual motors require a more careful determination of power requirement.
Generators.—Electric generators are shunt or compound wound. The latter can be made to give more uniform voltage. To operate compound wound generators together (feeding the same wires) an equalizer is used. This makes a common connection to all machines at a point between the armature and the series field and divides the series field currents in proper proportions between the machines. Generators to be operated together must have similar electrical characteristics. Voltage is proportional to revolutions per minute X number of poles X magnetic flux per pole. Generators (and motors) are rated as to capacity with reference to the rise of temperature in a stated length of time. In common practice a machine will not heat more than 90°F. above a surrounding air at 77°F. when running continuously at rated load. Regulation refers to variation of speed or voltage over a range of loads (usually no load to full load) and is expressed as a percentage of the full-load value. Full-load efficiencies of direct-current generators are from 0.80 to 0.93 for sizes from 1 to 500 hp. The variation of efficiency with load is about the same as for shunt motors. Weights are almost directly proportional to output. Costs per pound decrease somewhat as the size increases. High-speed (revolutions per minute) leads to low cost per kilowatt of capacity, but the variation in cost is less rapid than the change in speed.
Alternating-current Generators.—These may be of synchronous or induction type (some should always be of the former). Most large or high-voltage induction alternators are built with the fields rather than the armature revolving. Alternators may be single-phase, two or three-phase. The first are 30 per cent heavier and more costly and less efficient than the others: three-phase current is most economical of copper for transmission. Generator voltages range up to 13,000. Frequency —' revolutions per second X number of poles -f- 2. Frequencies of 25 and 60 cycles are standard. The latter is desirable for lighting or where static transformers are used. The former leads to reduced inductive voltage drop and is therefore better for long transmissions: it adds considerably to generator cost. It is to be preferred for frequent overloads and high starting torques in motor applications, or where synchronous motors or rotary transformers are to be driven. Alternators are rated (at 100 per cent power factor) on the basis of temperature rise: for example, 90° may be the limit at rated load, starting from 77°; or 99° for a 25 per cent overload lasting 2 hr. and immediately succeeding a long run at rated load. Polyphase alternators have full-load efficiencies from 91 to 97 per cent, increasing with size. Weights increase less rapidly than outputs: costs per unit of capacity decrease as capacity increases. Large units run at the lower speeds. For a given size, high speed means low cost. Turbine-driven machines operate at particularly high speeds and are so compact that special provision must be made for ventilation.
For the operation of alternators in parallel, the supply of power from the respective prime movers must be determined by governors having the same speed-load character-